1. Field of the Invention
This invention relates to the field of methods and apparatus for molding molten material, and in particular provides a mold structure and method for reducing and preferably minimizing the time needed to cool the molten material to a temperature at which the molded article is rigid enough for removal from the mold. This is accomplished by controlling the rate of heat transfer from the molten material to the mold body using thermally insulating surface temperature boosters. According to the invention, thermal insulation is embodied according to its thickness and heat transfer properties to maximize the rate of cooling within certain limits such that the material cools substantially to its solidifying temperature promptly upon completion of filling of the mold.
2. Prior Art
Production molding processes may produce hundreds of molded articles an hour. Most of these processes apply pressure to cause a hot molten material to flow into and fill a mold cavity. The material cools in the shape of the cavity until it is rigid enough for removal before the mold opens and the operator or special equipment removes the article from the mold.
Production efficiency dictates that each molding operation should be completed in as short a time as possible in order to make the mold available for another molding cycle. For this reason, molders normally try to cool the molten material as fast as possible. Often heat transfer fluid is circulated through passages in the mold dies to control the cavity surface temperature. U.S. Pat. Nos. 4,275,864, 4,655,280, 4,703,912, and 4,934,918 describe some ways to provide flow passages. Heat flows from the molten material through the mold die to the heat transfer fluid. Metals such as H-13, H-23, P-1, P-2, P-4, P-5, P-6, P-20, and S7 tool steels, 420 stainless steel, beryllium copper, brass, and aluminum are common mold materials whose high thermal conductivities cause heat to flow at a high rate. Irvin I. Rubin on page 156 of "Injection Molding Theory and Practice," explains why high thermal conductivity materials should be chosen for molds.
To remove heat from the molten material rapidly, molders normally keep cavity surfaces much colder than the molten material throughout the molding cycle. However, if cavity surfaces are kept too cold while the mold is filling, the mold may not fill completely (short shot) or unacceptable surface defects or stresses may develop in the molded article. In addition, locations distant from the location where the molten material enters the cavity may get less material than closer locations. This causes uneven density distribution and molded-in stresses.
The molten material tends to heat the mold. However, if the mold is kept at a relatively higher temperature, the mold generally can be filled more dependably because the molten material is more flowable, and the quality of the molded article is improved. The need for additional cooling can add to the time spent in molding each article as compared to keeping the mold cooler initially. What is needed is a method to optimize temperature control to balance the interests of quality and time for a given molding process.
The minimum cavity surface temperature required during mold filling depends on the particular molten material and the surface quality and dimensional stability required of the molded article. Processing temperature ranges for molten material and for mold dies are specified by the equipment and material manufacturers. For plastics, the recommended temperature ranges for mold dies are below the solidifying temperatures of the plastics. For many materials recommended temperature ranges can also be found in sources such as "Modern Plastics Encyclopedia," MIL-HDBK-700A "Plastics," the annual "Materials Selector" issues of "Materials Engineering Magazine," "Metals Handbook," "Glass Engineering Handbook," and "Kirk-Othmer: Encyclopedia of Chemical Technology Volume 11, third edition" (for glass see pages 825-832 & 855-857).
Because defects that may be acceptable for one type of molded article may not be acceptable for another, and because mold heating and cooling configurations vary, the optimum temperature for molding a specific article normally is determined in part by analysis and in part by experiment and experience (i.e., trial and error). Most often the optimum temperatures fall within the temperature ranges recommended by the material manufacturer.
When a molten material contacts surfaces of a cavity, heat flow from the molten material causes a rapid increase of the cavity surface temperatures. For example, molten polycarbonate at 600.degree. F. and cavity surfaces initially at 195.degree. F. will produce the following approximate temperature increases for cavity surfaces made of common mold die metals. These increases are approximate because convection, radiation, thermal contact resistances, changes in thermal physical properties with temperature, and initial temperature gradients can vary.
______________________________________ Cavity Surface Material Temperature Increase ______________________________________ 420 stainless steel 29.degree. F. H-13 tool steel 26.degree. F. brass 14.degree. F. aluminum 12.degree. F. ______________________________________
The small temperature increases at cavity surfaces of common metal cavities demand that molders maintain the surfaces of the cavity close to the mold filling temperatures throughout the molding cycle. Otherwise, the required temperatures cannot be reached at the cavity surfaces while the mold is filling. However, keeping the cavity surfaces so close to the mold filling temperatures after the cavity is full slows heat transfer from the molten material into the dies and delays solidifying of the workpiece. If die temperature is cycled instead, heating and cooling the entire mass of dies requires additional time that also slows the molding process.
The cavity surface temperature and the rate of cooling affect the finished workpiece. It is know, for example, deliberately to increase cooling time to improve surface qualities of the molded article such as smoothness, gloss, and replication of cavity surface finish.
DuPont Company developed a method for cycling the temperature of mold dies to improve the smoothness of molded surfaces. It is described in the article, "Class "A" Blow Molding: How It's Done," Plastics Technology, June, 1988. The method reduces the thermal mass of the mold dies then alternately circulates heating and cooling fluid through passages in the dies. This requires extensive structural analysis and machining in addition to computer controlled dual fluid circuits. DuPont reports that the process meets its goal to improve the surface of molded automobile spoilers. However, DuPont also states it increases the cycle time.
Others improve the surface quality of plastic articles by heating a thin layer of the cavity surface rather than the entire die. U.S. Pat. No. 4,340,551 discloses high-frequency induction to heat a superficial layer of the cavity surface before injecting plastic resin. Steps to insert the induction heater, close the mold, heat the cavity surface, open the mold, and remove the induction heater increase the cycle time. U.S. Pat. Nos. 3,734,449, 4,285,901, 5,041,247, and 5,064,597 locate a thin layer of metal backed by a thermal insulation layer at the cavity surface. The insulation layer reduces heat flow from the metal layer. The result is a substantially higher temperature of the metal cavity surface, so that it is above the point at which the resin solidifies while the mold cavity is filling. The high temperature and restricted heat transfer keep the surface of the plastic article fluid and improve transfer of the finish of the very hot surface of the cavity to the plastic article. The increased cavity surface temperature and restricted heat transfer increase cooling time. M. Liou and N. Suh in their article, "Minimizing Residual Stresses in Molded Parts," pages 524 through 528, ANTEC '88, report that coating a cavity surface with 0.01 centimeter of Teflon caused higher cavity surface temperatures that increased cooling time almost twenty percent.
To reduce cooling time, inventors have applied cooling fluid directly to the surface of a mold cavity. U.S. Pat. Nos. 4,139,177 and 4,164,523 reduce the cooling time of thick foam articles by flowing low boiling point liquid between the article and the cavity surface. The compressibility of foam allows space for the liquid to flow. The method is limited to molding foamed plastic articles, and at least two of the preferred liquids, namely carbon dioxide and liquid nitrogen, can pose safety hazards. The extreme cold temperatures of these liquids can cause skin damage, and the gases from the boiling liquid can displace air in the work place. U.S. Pat. No. 4,208,177 flows cooling fluid at or about five pounds per square inch pressure into a porous layer at the cavity surface. A vacuum is then pulled on the side of the porous layer that is away from the resin. The vacuum causes the fluid to boil, which draws heat from the porous layer and resin. However, the pores limit the quality of surface finishes on the molded article. Methods that rely on direct contact of low boiling point fluid with the cavity surfaces require expensive equipment and complicated controls.
In summary, the teachings of the prior art:
a. maintain the cavity surfaces close to the temperatures required during mold filling throughout the molding cycle, which prolongs cooling; or, PA1 b. to improve surface quality of a molded article, increase the temperature of cavity surfaces above the solidifying temperature of a plastic being molded while the cavity is filling, which lengthens cooling time even more; and/or, PA1 c. cycle the temperatures of entire mold dies, which requires expensive and complicated equipment; or, PA1 d. accelerate cooling by bringing fluid into direct contact with cavity surfaces which requires complex equipment and can degrade the molded article. PA1 (a) providing a mold containing a plurality of mold portions which are brought together to form a mold cavity in the shape of said molded article, said mold portions comprising: PA1 (b) applying substantially constant temperature control stimuli to said mold dies such that surfaces of the mold cavity are at predetermined temperatures that are initially below the temperatures required to produce a molded article and which will, upon contact with molten material to be introduced into the mold cavity, increase to or above the temperatures required during mold filling to produce a molded article, and wherein, because of the mathematical products of thermal conductivity, density, and specific heat of the primary boosters the predetermined cavity surface and die temperatures are lower than when materials with higher corresponding products are used for cavity surfaces, such as when the same die is used without the primary boosters; PA1 (c) introducing molten material into the mold cavity, whereupon the molten material heats the primary boosters and temperatures of the surfaces of the cavity increase from the predetermined temperatures to or above the mold filling temperatures required to produce a molded article; PA1 (d) while the cavity is filling with molten material, maintaining said mold cavity surfaces at or above the temperatures required to produce a molded article; PA1 (e) after the cavity is approximately full, allowing heat flowing from the boosters to the cooler dies to cool cavity surfaces of the primary boosters, thereby cooling, stiffening, and solidifying the molten material in an accelerated manner, until it is rigid enough for removal from the mold.
The extremely cold liquids used for one of these methods present safety hazards and the method is limited to foamed resin.